This invention relates generally to acoustic droplet emission and more particularly concerns a capping structure which provides liquid level control and meniscus placement for an acoustic droplet emitter.
Turning now to FIG. 1 a device which generates liquid droplets using focussed acoustic energy is shown. Such devices are known in the art for use in printing applications. Detailed descriptions of acoustic droplet formation and acoustic printing can be found in the following U.S. patent applications Ser. No. 4,308,507 titled "Liquid Drop Emitter" by Lovelady et al., issued Dec. 29.sup.th, 1981; U.S. patent application Ser. No. 4,697,195 titled "Nozzleless Liquid Droplet Ejectors", by Quate et. al., issued Sep. 29.sup.th, 1987; U.S. patent application Ser. No. 5,041,849 titled "Multi-Discrete-Phase Fresnel Acoustic Lenses And Their Application To Acoustic Ink Printing" to Quate et al., issued Aug. 20.sup.th, 1991; U.S. patent application Ser. No. 5,121,141 titled "Acoustic Ink Printhead With Integrated Liquid Level Control Layer" to Hadimioglu et al., issued Jun. 9.sup.th, 1992; U.S. patent application Ser. No. 5,608,433 titled "Fluid Application Device And Method Of Operation" by Quate, issued Mar. 4.sup.th, 1997, all herein incorporated by reference, as well as other patents.
The most important feature of the device shown in FIG. 1 is that it does not use nozzles and is therefore unlikely to clog, especially when compared to other methods of forming and ejecting small, controlled droplets. The device can be manufactured using photolithographic techniques to provide groups of densely packed emitters each of which can eject carefully controlled droplets. Furthermore, it is known that such devices can eject a wide variety of materials, U.S. Pat. No. 5,591,490 titled "Acoustic Deposition Of Material Layers" by Quate, issued Jan. 7.sup.th, 1997 and herein incorporated by reference, describes a method for using an array of such acoustic droplet emitters to form a uniform layer of resist, U.S. Pat. No. 5,565,113 titled
"Lithographically Defined Ejection Units" by Hadimioglu et al., issued Oct. 15.sup.th 1996, and herein incorporated by reference, states that the principles of Acoustic Ink Printing(AIP) are suitable for ejection of materials other than marking fluids, such as mylar catalysts, molten solder, hot melt waxes, color filter materials, resists, chemical compounds, and biological compounds. U.S. Pat. No. 5,520,715 titled "Directional Electrostatic Accretion Process Employing Acoustic Droplet Formation" by Oeftering, issued May 28.sup.th, 1996, and herein incorporated by reference describes using focussed acoustic energy to emit droplets of liquid metal.
With the above concepts firmly in mind, the operation of an exemplary acoustic droplet emitter will now be described. There are many variations in acoustic droplet emitters and the description of a particular droplet emitter is not intended to limit the disclosure but to merely provide an example from which the principles of acoustic droplet generation can be applied in the context of this invention.
FIG. 1 shows an acoustic droplet emitter 10 shortly after emitting of a droplet 12 of a liquid 14 and before a mound 16 on a free surface 18 of the liquid 14 has relaxed. The forming of the mound 16 and the subsequent ejection of the droplet 12 is the result of pressure exerted by acoustic forces created by a ZnO transducer 20. To generate the acoustic pressure, RF energy is applied to the ZnO transducer 20 from an RF source 22 via a bottom electrode 24 and a top electrode 26. The acoustic energy from the transducer 20 passes through a base 28 into an acoustic lens 30. The acoustic lens 30 focuses its received acoustic energy into a small focal area which is at or very near the free surface 18 of the liquid 14. It should be noted that while the acoustic lens 30 is depicted as a fresnel lens, that other lenses are also possible. For example, concave acoustic beam forming devices such as that shown in U.S. Pat. No. 4,751,529, titled "Microlenses For Acoustic Printing", by Elrod et al., issued Jun. 14.sup.th, 1988 have also been used. Provided the energy of the acoustic beam is sufficient and properly focused relative to the free surface 18 of the liquid 14, a mound 16 is formed and a droplet 12 is subsequently emitted on a trajectory T.
The liquid is contained by a plate 34 which has a opening 32 in which the free surface 18 of the liquid 14 is present and from which the droplet 12 is emitted. The liquid 14 flows through a channel defined by sidewalls 36 and the top surface 38 of base 28 and past the acoustic lens 30 without disturbing the free surface 18. Although the sidewalls 36 are depicted as inwardly sloping, resulting in a channel that is narrower at the opening 32 than at the surface 38 of the base 28, this need not be so. Examples of other channel configurations are shown in U.S. Pat. No. 5,121,141, issued Jun. 9.sup.th, 1992, by Hadimioglu et al., and titled, "Acoustic Ink Printhead With Integrated Liquid Level Control Layer" and U.S. Pat. No. 5,450,107, issued Sep. 12.sup.th, 1995, by Rawson and titled "Surface Ripple Wave Suppression By Anti-Reflection In Apertured Free Ink Surface Level Controllers For Acoustic Ink Printers", both herein incorporated by reference. The width W of the opening 32 is many times larger than the droplet 12 which is emitted such that the width W of the opening has no effect on the size of the droplet 12 thereby greatly reducing clogging of the opening, especially as compared to other droplet ejection technologies. It is this feature of the droplet emitter 10 which makes its use desirable for emitting droplets of a wide variety of materials. Also important to the invention is the fact that droplet size of acoustically generated and emitted droplets can be precisely controlled. Drop diameters can be as small as 16 microns allowing for the deposition of very small amounts of material.
However, the free surface 18 of the liquid 14 must be a precise focal distance d from the acoustic lens 30 so that the acoustic energy focussed by the acoustic lens 30 can be focussed at or very near to the free surface 18. Variations in the distance d will cause the acoustic energy generated by the transducer 20 to be misfocused by the acoustic lens 30 and often results in misfired droplets 12. Many techniques have been used to control the placement of the free surface 18 relative to acoustic lens 30.
Most commonly, surface tension, fluid pressure, and the edge of an orifice opening are relied upon to place the free surface 18 at the appropriate distance d. If the liquid 14 is supplied at the correct pressure then the surface tension will hold the free surface 18 in place with a meniscus extending between the sidewalls 36, as shown in FIG. 1. If the pressure is increased the liquid 14 will spill through the opening, if the pressure is decreased the free surface 18 of the liquid 14 will slip down the sidewalls 36 of the plate 34 instead of being adjacent to the top surface 40 of the plate 34 as shown in FIG. 1.
This method requires uniformity of the pressure of liquid 14 and is dependent on variations in the thickness of the plate 34. In the case of an acoustic droplet emitter which has a single emitter or a small number of emitters, pressure uniformity can often be sufficiently maintained. However, as the number of emitters disposed in a single channel grow larger, maintaining uniformity can be problematic. Furthermore, the free surface may not be maintained by the sidewalls of the channel but by the sidewalls of a relatively short capping structure as shown in any of U.S. Pat. No. 5,121,141 titled "Acoustic In Printhead With Integrated Liquid Level Control Layer" to Hadimioglu et al., issued Jun. 9.sup.th, 1992, U.S. Pat. No. 5,450,107, titled "Surface Ripple Wave Suppression By Anti-Reflection In Apertured Free Ink Surface Level Controllers For Acoustic Ink Printers", by Rawson, issued Sep. 12.sup.th, 1995, U.S. Pat. No. 5,028,937, titled "Perforated Membranes For Liquid Contronlin Acoustic Ink Printing", by Khuri-Yakub et al., issued Jul. 2.sup.nd, 1991, U.S. Pat. No. 5,121,141 titled "Acoustic In Printhead With Integrated Liquid Level Control Layer" to Hadimioglu et al., issued Jun. 9.sup.th, 1992, or U.S. Pat. No. 5,216,451, titled "Surface Ripple Wave Diffusion In Apertured Free Ink Surface Level Controllers For Acoustic Ink Printers", by Rawson et al., issued Jun. 1.sup.st, 1993, Incorporated by reference hereinabove. In these cases, if the pressure drops too low, the free surface will drop below the level of the capping structure and the system will begin to take in air.
Another method has been shown in U.S. Pat. No. 5,277,754, titled "Process For Manufacturing Liquid Level Control Structure" by Hadimioglu et al., issued Jan. 11.sup.th, 1994, and U.S. Pat. No. 5,392,064 titled "Liquid Level Control Structure" by Hadimioglu et al., issued Feb. 21.sup.st, 1995, both incorporated by reference hereinabove. These patents describe an hourglass-shaped aperture containing knife edged lips at the waist of the aperture. While this embodiment has the advantage of being independent from variations in wafer thickness it is difficult to manufacture and not as easily extensible to larger numbers of emitters.
Further work has been done in the area as shown in U.S. Pat. No. 5,277,754, titled "Process For Manufacturing Liquid Level Control Structure" by Hadimioglu et al., issued Jan. 11.sup.th, 1994, and U.S. Pat. No. 5,392,064 titled "Liquid Level Control Structure" by Hadimioglu et al., issued Feb. 21.sup.st, 1995, both incorporated by reference hereinabove. Structures are shown which utilize acoustically thin capping structures having pores to create accurately positioned fluid wells. As above, these structures are complicated to manufacture and are dependent on variations in thickness of both the substrate and the capping structures.
Accordingly, it is the primary aim of the invention to create a method for precise placement of a liquid with a free surface that is easy to manufacture, easily extensible to many emitters within a single channel, (enabling a high rate of flow of the liquid) and has as few dependencies as possible on thickness variations of various components.
Further advantages of the invention will become apparent as the following description proceeds.